Abstract: The thermal stratification phenomenon in the non-isolated piping of pressurized water reactors (PWRs) can lead to thermal fatigue and structural deformation of the piping. In severe cases, pipe rupture failure may be induced, thereby threatening the safe operation of the reactor. To investigate the thermal stratification phenomenon in the suction pipe of the passive residual heat removal system (PRHRS) of a small modular reactor (SMR), the conjugate heat transfer between the solid and fluid was taken into account, and numerical simulation of the flow and heat transfer in the suction pipe was conducted. The axial temperature distribution of the pipe and the thermal stratification characteristics at the pipe cross-sections were obtained. Results indicate that the pipe temperature distribution generally exhibits a gradually decreasing trend along the axial direction. Since the insulation layer ensures the driving force for natural circulation, a smaller axial temperature gradient is observed in the pipe region with insulation, whereas a larger axial temperature gradient is found in the pipe region without insulation. When the pipe outlet temperature is varied, the cross-sectional average temperature is found to vary consistently along the axial direction, while a more pronounced effect on the cross-sectional temperature difference is exerted on the insulated pipe region. Under leakage conditions, as the inlet flow velocity is increased, the cross-sectional average temperature is elevated, the cross-sectional temperature difference in the insulated region is reduced, while that in the uninsulated region is increased, resulting in intensified thermal stratification. Relevant simulation results can provide a theoretical basis for the design and analysis of SMRs.